UMTS (Universal Mobile Telecommunications System) is the 3G (3rd Generation) mobile communication system standardised by 3GPP (3rd Generation Partnership Project). The first release of the specification of UMTS was published in 1999 (Release 99). In the mean time several improvements to the standard have been standardized by the 3GPP in Release 4, Release 5 and Release 6. With the desire to support higher data rates, it was decided to develop a new Air Interface and also a new evolved radio access network, E-UTRAN (UMTS Terrestrial Radio Access Network). The 3GPP launched a study item “Evolved UTRA and UTRAN” better known as “Long Term Evolution (LTE)”. The study will investigate means of achieving major leaps in performance in order to improve service provisioning, and to reduce user and operator costs. Out of that and because interworking with other radio access technologies should be possible, the need arose for a new evolved Packet Core Network.
The E-UTRAN architecture consists of evolved Node Bs (eNB or eNodeB), providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the mobile node, as explained below.
The eNB hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC), and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. Further, it performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated UL-QoS (UpLink-Quality of Service), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of DL/UL (DownLink/UpLink) user plane packet headers. The eNBs are interconnected with each other by means of the X2 interface. The eNBs are also connected by means of the S1 interface to the EPC (Evolved Packet Core), more specifically to the MME (Mobility Management Entity) by means of the S1-MME, and to the Serving Gateway (S-GW) by means of the S1-U. The S1 interface supports a many-to-many relation between MMEs/Serving Gateways and eNBs.
The S-GW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and Packet Data Network Gateway). For idle state UEs, the S-GW terminates the DL data path and triggers paging when DL data arrives for the UE. It manages and stores UE contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.
The MME is the key control-node for the LTE access-network. It is responsible for idle mode UE tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the S-GW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the Home Subscriber Server, HSS). The Non-Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service provider's Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN (Serving GPRS Support Node). The MME also terminates the S6a interface towards the home HSS for roaming UEs.
The Packet Data Network Gateway (PDN-GW) provides connectivity for the UE to external packet data networks by being the point of exit and entry of traffic for the UE. A UE may have simultaneous connectivity with more than one PDN-GW for accessing multiple PDNs. The PDN-GW performs policy enforcement, packet filtering for each user, charging support, lawful Interception and packet screening. Another key role of the PDN-GW is to act as the anchor for mobility between 3GPP and non-3GPP technologies.
To summarize the above, in order to support the new E-UTRAN access, the new 3GPP Core Network is mainly separated into three logical entities. At first, in the user plane the PDN-GW is the gateway to the external networks and the global mobility anchor for mobility between 3GPP and non-3GPP access technologies (like CDMA2000, WiMAX or WIFI). Second, another user plane entity being the Serving Gateway is the mobility anchor for mobility between 3GPP accesses (E-UTRAN, UTRAN, GERAN). Thirdly, a Mobility Management Entity is the control plane entity responsible for the mobility management of mobile terminals (also referred to in the following as UEs or MNs) moving between different EUTRAN base stations (eNodeBs) and also responsible for the session management.
The MMEs are connected to the eNodeBs, and one MME might be serving a number of eNodeBs so that multiple MMEs are necessary within the system to cover all eNodeBs. Furthermore, a pool of MMEs might be serving the same set of eNodeBs, e.g. for load balancing reasons.
As described above, the MME is responsible for mobility management and session management. For each mobile terminal attached to an MME, specific mobility management and evolved packet system context information is stored in the MME. These contexts comprise, e.g. the mobility state, the temporary identity, the current Tracking Area List, last known cell, authentication vectors, access restrictions, subscribed QoS profile, subscribed charging characteristics, and for each active PDN connection the APN (Access Point Name) in use, IPv4/IPv6 addresses, PDN-GW address for control plane, and also information for each EPS (Evolved Packet System) bearer within the PDN connection, as for example EPS bearer QoS profile, EPS bearer charging characteristics.
Furthermore, the context for a mobile terminal in an MME might be available even if the mobile terminal is detached from the 3GPP access. This context preservation allows for faster session setup when again switching on in the 3GPP access or when handing over from a non-3GPP access back to the 3GPP access, mainly because signalling with the Home Subscriber Server (HSS) is saved.
The following two sections give some additional background information about two important topics in this invention: Identities and context management.
UE Identifiers
Each UE can be identified by its IMSI, which is a permanent identity of the subscriber and this is a globally unique identifier.
Next to the IMSI, each UE is assigned a Globally Unique Temporary Identity (GUTI) by the MME. The GUTI is used to support subscriber identity confidentiality, and, in the shortened S-TMSI form, to enable more efficient radio signalling procedures (e.g. paging and Service Request).
The GUTI has two main components:                one that uniquely identifies the MME which allocated the GUTI; and        one that uniquely identifies the UE within the MME that allocated the GUTI.        
UE Context
FIG. 1 illustrates the context management procedure. While the UE is connected, the context of the UE at the MME is always synchronized with the UE's data at the HSS. After the UE detaches, the MME receives a notification about it, and now has the option to keep the UE's context for a specific amount of time. The benefit of keeping the context at the MME is that if the UE reattaches to the network, the data at the MME can be reused.
While keeping the context at the MME, every change to the subscription info of the UE which is stored at the HSS (and of importance to the operation at the MME) gets updated to the MME. At the moment the MME decides to delete the context, it sends a “purge” message to the HSS, informing the HSS that it is about the delete the context. After receiving this message at the HSS, the HSS stops sending updates to the context at the MME. Note that the UE does not know about the existence of context at the MME.